Google's Pixel line of smartphones has always been plagued with some issues or the other, and it's no different for the Pixel 4 too. The latest issue appears to affect the white balance correction algorithm in the camera app, which according to users, drastically changes the colour tone of the image when shooting under artificial light. This majorly seems to affect reds, where red artificial lighting ends up looking yellow in the final photo. It could be a rare case, where the white balance system is malfunctioning due to the type or intensity of the light, but it's an issue which shouldn't happen nonetheless. Google is yet to officially respond to this.
Reddit user ‘nalrodriguez' posted a thread about this issue, where he managed to replicate it on three different Pixel 4 XLs. He had changed the lighting in his house to bright red, using Philips Hue lights, for Halloween and when he went to take a picture, the final images were yellow instead of have a red tone. He also mentioned that it turned some of the purple lights into blue. Here's a photo gallery he uploaded with all the photos. He mentions that this issue isn't happening with his Pixel 3 (Review) smartphone and RAW images have the correct red colour tone. This means it's the new HDR+ processing in the Pixel 4 XL that's probably aggressively trying to ‘correct' the white balance.
There haven't been any other reports of this extreme behaviour so far but, the algorithm in the Pixel 4 might have an issue rendering reds correctly, especially when shot under artificial lighting. Ryne Hager from Android Police, recently faced something similar when he shot an image of a deep red cinema curtain, only to find the result having a pale red colour tone instead. Android Police mention that it's not conclusive it this was a Pixel 4-specific issue but considering the issue posted on Reddit, it's highly possible.